The human eye is a delicate organism that’s difficult to copy. But, a team of engineers at Northwestern University and the University of Illinois hope to mimic the eye in a new type of camera lens they are developing. They have created an array of silicon detectors and electronics that can be conformed to a curved surface.
The human eye is a delicate organism that’s difficult to copy. But, a team of researchers at Northwestern University and the University of Illinois hope to mimic the eye in a new type of camera lens they are developing.
The engineers have created an array of silicon detectors and electronics that can be conformed to a curved surface. Like the human eye, the curved surface can then act as the focal plane array of the camera, which captures an image. The goal is to produce photographic images with a wider field of view.
On a traditional camera, such electronics must lie on a straight surface, and the camera’s complex system of lenses must reflect an image several times before it can reflect on the right spots on the focal plane.
“The advantages of curved detector surface imaging have been understood by optics designers for a long time, and by biologists for an even longer time,” says Yonggang Huang, a professor of civil, environmental and mechanical engineering at Northwestern University. “That’s how the human eye works-using the curved surface at the back of the eye to capture an image.”
But, exactly how to place those electronics on a curved surface to yield working cameras has stumped scientists, despite many different attempts over the last 20 years. The electronics lie on silicon wafers, which can only be compressed 1 percent before they break and fail. Huang and his colleagues have established experimental methods and theoretical foundations for an effective way to transfer the electronics from a flat surface to a curved one.
To address the issue, John Rogers, a professor of materials science and engineering at the University of Illinois, created a hemispherical transfer element made out of a thin elastomeric membrane that can be stretched out into the shape of a flat drumhead. In this form, planar (flat) electronics can be transferred onto the elastomer. Popping the elastomer back into its hemispheric form enables the transfer of the electronics onto a hemispherical device substrate.
When such a process is applied to conventional electronics, it leads to catastrophic mechanical fracture in the brittle semiconductor materials. Rogers and Huang got around this challenge by creating an array of photodetectors and circuit elements that are so small-approximately 100 micrometers square-they aren’t as affected when the elastomer pops back into its hemispheric shape.
In addition, each of these devices on the array is connected by thin metal wires on plastic, which form arc-shaped structures that Huang and Rogers call “pop-up bridges.” The bridges interconnect the silicon devices, thereby relaxing all of the strain associated with return of the elastomer to its curved shape.
The engineers also designed the array so that the silicon component of each device is sandwiched in the middle of two other layers, the natural mechanical plane. That way, while the top layer is stretched and the bottom layer is compressed, the middle layer experiences very small stress.
Early images obtained using this curved array in an electronic eye-type camera indicate large-scale pictures that are much clearer than those obtained with similar, but planar, cameras, when simple imaging optics are used.
“In a conventional, planar camera, parts of the images that fall at the edges of the fields of view are typically not imaged well using simple optics,” claims Huang. “The hemisphere layout of the electronic eye eliminates this and other limitations, thereby providing improved imaging characteristics.”